What admixtures are added to concrete mixes used in NB to improve resistance to freeze-thaw damage?

The single most important admixture for freeze-thaw resistance in New Brunswick concrete is an air-entraining agent — and for exterior flatwork in NB, it is non-negotiable.

Air-entraining admixtures work by introducing billions of microscopic, uniformly distributed air bubbles into the concrete matrix during mixing. These bubbles — typically achieving 4–7% total air content by volume — act as pressure relief valves. When water penetrates the concrete and freezes, it expands by roughly 9%. Without those air voids, that expansion has nowhere to go and fractures the concrete from within. With proper air entrainment, the freezing water migrates into the nearest air bubble and expands safely. Over New Brunswick's 150+ freeze-thaw cycles per year, the difference between air-entrained and non-air-entrained concrete is the difference between a driveway that lasts 30 years and one that begins spalling within 3–7 years.

Air-entraining agents are typically added at the ready-mix plant as a liquid admixture (common products include Darex AEA, MB-AE 90, and similar CSA A266.1-compliant agents). When ordering ready-mix in NB, always specify air-entrained mix with 5–7% air content for driveways, patios, sidewalks, steps, and any other exterior flatwork. Most NB ready-mix suppliers default to air-entrained mixes for exterior work, but confirm it explicitly — never assume.

Water reducers (plasticizers) are the second most important admixture for NB freeze-thaw performance, though they work indirectly. A lower water-to-cement ratio produces denser, less permeable concrete with fewer capillary pores for water to enter in the first place. Mid-range water reducers allow you to maintain workability at a 4–5 inch slump while keeping the water-to-cement ratio at or below 0.45 — the threshold generally recommended for freeze-thaw exposed concrete under CSA A23.1. This is far preferable to adding water at the jobsite, which weakens the mix and opens up the pore structure.

Supplementary cementitious materials (SCMs) — particularly silica fume and fly ash — are used in higher-performance NB mixes, especially for coastal applications around Saint John, Shediac, and the Bay of Fundy communities where chloride penetration compounds freeze-thaw damage. Silica fume fills microscopic voids in the cement paste, dramatically reducing permeability. Fly ash (Class F) improves long-term strength and reduces the heat of hydration. These are more common in commercial and infrastructure work but are available through NB ready-mix suppliers for residential projects where durability is a priority.

Accelerating admixtures (calcium chloride or non-chloride alternatives like calcium nitrite) are used in NB for late-season pours — October and November — to speed up strength gain before overnight temperatures drop toward freezing. However, calcium chloride should never be used in reinforced concrete — it accelerates corrosion of the rebar. Non-chloride accelerators are the correct choice for any reinforced slab, foundation, or structural element.

A few practical points for NB homeowners and contractors:

Specify the full mix design when ordering, not just the strength. Ask for: air-entrained, 25–32 MPa, maximum 0.45 water-to-cement ratio, 4–5 inch slump. For coastal locations, request 32 MPa minimum.

Pair admixtures with a penetrating silane/siloxane sealer applied after the concrete has cured for at least 28 days. Admixtures improve the concrete's internal resistance; the sealer reduces how much water gets in to begin with. Together, they give NB exterior concrete its best chance at a long service life.

Fibre reinforcement (polypropylene or steel fibres) is sometimes added to reduce plastic shrinkage cracking during the early curing stage, but fibres do not improve freeze-thaw resistance — that job belongs to air entrainment.

If you're planning an exterior concrete project this season, New Brunswick Concrete can match you with local contractors who understand NB mix specifications and will order the right product for your conditions — the matching service is completely free.